1. Field of the Invention
The present invention relates to a method of and an apparatus for generating a halftone dot image having a desired screen ruling.
2. Description of the Related Art
In the fields of desktop printing and printing/platemaking technology, there has been employed an image processing system which reads a color subject with a scanner to obtain input image data, effects a desired image processing process on the input image data to produce output image data, and if the image data is a continuous gradation image data, converts the image data into halftone dot image data, which is supplied to an image output device such as a plotter, a printer, or the like to output an image on a recording medium.
The image processing system is constructed of various devices. Because of a recent rapid progress toward open industry standards for such devices, various devices manufactured by different manufacturers can be connected together to realize the image processing system. For example, any of various different image output devices can be connected to an image binarizer which converts multivalued image data into binary image data for thereby generating a halftone dot image.
Different image output devices tend to have different resolutions. If image data is reproduced by different image output devices having different resolutions, then some halftone dot images cannot be generated with a desired screen ruling.
For example, it is assumed that a halftone dot image with a screen ruling of 100 LPI (Lines Per Inch) is to be outputted. If an image output device having a resolution of 1000 DPI (Dots Per Inch) is used to output a halftone dot image, then an image binarizer generates image data of 10xc3x9710 dots per halftone dot, and the image output device outputs a halftone dot image based on the image data from the image binarizer. If an image output device having a resolution of 800 DPI (Dots Per Inch) is used, it outputs a halftone dot image having a screen ruling of 80 LPI. As described above, when the same threshold data is employed to generate binary image data from the image binarizer, if an image output device having a different resolution is used to output a halftone dot image based on the binary image data, then the halftone dot image has a different screen ruling, and hence is different from a desired image.
The above problem can be solved if the image binarizer has a number of sets of threshold data in order to cope with the resolutions and output image screen rulings of different image output devices that can possibly be connected to the image binarizer. However, the image binarizer needs to have a sufficiently large memory capacity for storing such a number of sets of threshold data, and it takes a long period of time to generate those sets of threshold data.
It is a general object of the present invention to provide a method of and an apparatus for generating a halftone dot image having a desired screen ruling with a reduced amount of threshold data that are generated in a reduced period of time and stored in a reduced storage capacity, while coping with the different resolutions of various image output devices.
Another object of the present invention is to provide a method of and an apparatus for generating a halftone dot image having a screen ruling which is closest to a desired screen ruling.